Holes of intermediate diameter are usually produced in solid materials by the use of either twist or spade drills. Drill performance is vastly improved by the use of cutting inserts of extremely hard alloy, and these are difficult to install and maintain on the usual twist drill. The spade drill has no helical configuration, and therefore can make practical use of the so-called indexable insert that provides four cutting edges around the periphery of a square configuration. Any one of these edges can be positioned to do the cutting, and a central hole in the insert provides for a fastening holding the insert in a recess in a close fit.
Problems develop when one of these relatively brittle inserts cracks, and then presents a blunt surface to the material being cut. Heavy torque and axial feed forces quickly damage the relatively expensive tool holder. The cost of these holders also inhibits the use of large numbers of these drills, which are otherwise the most economical devices for hole production in the diameter ranges where the indexable inserts can be accommodated. The performance and characteristics of this type of drill is discussed in the April, 1979, issue of MODERN MACHINE SHOP, beginning at Page 84.